linux-kernel - Re: [PATCH 08 of 11] anon-vma-rwsem

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Message-ID: <20080508013459.GS8276@duo.random>
Date:	Thu, 8 May 2008 03:34:59 +0200
From:	Andrea Arcangeli <andrea@...ranet.com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, clameter@....com,
	steiner@....com, holt@....com, npiggin@...e.de,
	a.p.zijlstra@...llo.nl, kvm-devel@...ts.sourceforge.net,
	kanojsarcar@...oo.com, rdreier@...co.com,
	swise@...ngridcomputing.com, linux-kernel@...r.kernel.org,
	avi@...ranet.com, linux-mm@...ck.org,
	general@...ts.openfabrics.org, hugh@...itas.com,
	rusty@...tcorp.com.au, aliguori@...ibm.com, chrisw@...hat.com,
	marcelo@...ck.org, dada1@...mosbay.com, paulmck@...ibm.com
Subject: Re: [PATCH 08 of 11] anon-vma-rwsem

Sorry for not having completely answered to this. I initially thought
stop_machine could work when you mentioned it, but I don't think it
can even removing xpmem block-inside-mmu-notifier-method requirements.

For stop_machine to solve this (besides being slower and potentially
not more safe as running stop_machine in a loop isn't nice), we'd need
to prevent preemption in between invalidate_range_start/end.

I think there are two ways:

1) add global lock around mm_lock to remove the sorting

2) remove invalidate_range_start/end, nuke mm_lock as consequence of
   it, and replace all three with invalidate_pages issued inside the
   PT lock, one invalidation for each 512 pte_t modified, so
   serialization against get_user_pages becomes trivial but this will
   be not ok at all for SGI as it increases a lot their invalidation
   frequency

For KVM both ways are almost the same.

I'll implement 1 now then we'll see...
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